There are many applications in which a driven device in the form of a pump, compressor, etc., is driven at a variable speed by an electric motor connected to a source of line power through a motor drive. Such a motor drive typically includes an input rectifier section and an output inverter section. The input rectifier section receives alternating current power at a constant frequency from the source of line power, and converts the alternating current received into a DC current supplied on a DC bus at the output of the rectifier section. The inverter section has an input connected to the DC bus, and is controlled to provide an alternating current motor drive signal at inverter outputs connected to the electric motor. The frequency and amplitude of the alternating current motor drive signal is adjusted as desired to cause the motor to run at a desired motor speed.
Such motor drive arrangements typically also include a power storage arrangement connected across the bus. The power storage arrangement typically includes a capacitor arrangement, and is configured to be charged and discharged by current flowing into the bus from the input rectifier section and out of the bus to the inverter section during operation of the motor drive in such a manner that the motor drive signal shape is improved. Another function of the power storage arrangement is to hold the voltage across the bus relatively constant during minor interruptions or other perturbations of the power being received from the line source.
For many applications, power storage arrangement can be configured to accommodate even somewhat extended interruptions of power from the line source by simply increasing the amount of capacitance in the power storage arrangement. There are practical limits inherent in this type of a motor drive arrangement, however, on the degree to which the capacitance in the power storage arrangement can be increased.
In applications of motor drives where a driven device exhibits a strong tendency to back drive the motor at high speed and/or torque, for example, the small to moderate capacitance that would typically be included in a power storage arrangement of a motor drive may not be sufficient to keep the driven device rotating in its normal drive direction for the full duration of an interruption to the line power. While it would seem to be logical to merely continue to increase the capacitance in the power storage arrangement to a degree necessary for sustaining continued operation of the motor in driving the driven device throughout the duration of the power interruption, there are a number of practical limitations and disadvantages to taking this approach.
Motor applications that are susceptible to having the motor be driven backwards with substantial speed and/or torque under power disturbance conditions include applications such as submersible pumps, or pumps and compressors supplying pressurized fluid to a storage tank located above the pump, or a pressurized tank or accumulator.
For example, where the motor is driving a submersible centrifugal pump located at the bottom of a wellbore of the type encountered in the oil industry, there may be a mile or more of fluid in a discharge line above the centrifugal pump impeller, between the pump and the surface of the ground above. When power to the centrifugal pump is lost through an interruption of electrical power supplied to the drive motor connected to the impeller, the weight of fluid in the discharge line above the pump impeller will cause the pump impeller to slow down and eventually reverse direction as the fluid in the discharge line is urged by gravitational force to flow backwards through the pump impeller into the wellbore. Where the quantity and weight, or pressure, of the fluid in the discharge line is substantial, this phenomenon can cause the centrifugal pump to be accelerated to a very high speed in the reverse direction. Once the impeller begins to rotate in an opposite direction, and particularly once it has begun to rotate at high speed, it may be impossible for the motor drive to regain control of the motor and return the impeller of the pump to its normal rotational direction following a restoration of input power.
If a submersible pump loses power from the mains long enough for the column of fluid to force the motor to spin in reverse, the pump cannot be restarted until the motor and pump come to rest. It can take a long time for the column of fluid to descend with the resulting loss of production. Compounding the loss of production problem, it can take a long time for the pump to refill the column of fluid after power is restored before actual production resumes. A large capacitor arrangement attached to the inverter's bus could theoretically keep the drive running while the mains are interrupted, but the current inrush when the mains are restored can destroy the rectifier section of many inverters. Inverters with small bus capacitance do not have the inrush problem, but the addition of a large capacitor arrangement would recreate the problem.
In addition to causing undesirably high inrush current, simply increasing the capacitance in the power storage arrangement can have other undesirable consequences. As will be appreciated by those having skill in the art, with relatively low capacitance in the power storage arrangement, there is a lower level of line harmonics. As the capacitance in the power storage arrangement is increased to a high level, however, the diodes in the rectifier section only conduct at the high end of the input voltage, and cause additional amplitude of the line harmonics which are problematic to the utility providing the source of line power. Such harmonics can rise to such a level that the utility would impose a surcharge for dealing with such harmonics. Further, in some cases where power from an electric utility grid is not available, generators, solar power and/or wind power may be used and such devices may be damaged or need to be oversize to deal with the effects of increased harmonics.
Another disadvantage of simply increasing the size of capacitors in the power storage arrangement is that, where the capacitance is relatively small, film capacitors can be utilized in the power storage arrangement. Such film capacitors have demonstrated good service life, even when subjected to significant current ripple resistance. Where a large capacitance is required, however, film capacitors do not provide adequate energy density and must be replaced by other types of capacitors, such as electrolytic capacitors. These capacitor types do not provide some of the desirable inherent characteristics of the film capacitors.
What is needed therefore, is an improved apparatus for operation of an electric motor driving a device that has a tendency to be driven backwards with substantial speed and/or torque under power disturbance conditions.